1. Field of the Invention
The present invention relates to a semiconductor power module which mounts a plurality of power semiconductor switches, such as insulated gate bipolar transistors (IGBTs), gate turn-off (GTO) thyristors and the like in a single package, and a large scale module comprising a plurality of the semiconductor power modules. More particularly, the present invention relates to the large scale module suitable for application fields in which various specifications are required as well as a high reliability for a long life. Especially, the present invention pertains to a power converter suitable for motion control of an electric locomotive, in which very severe reliability for a long life is substantially supposed in nature.
2. Description of the Related Art
As known in the art, the high frequency operation of the power converter can reduce the size and weight of the converter. And the power conversion at higher and higher frequencies is desired for power converters used in control systems for driving electric locomotives, since the compactness and the light weight of the converters suitable for installing in a railcar body are required to increase the transportation efficiency. And the high frequency power conversion of the power converters can simultaneously satisfy the comfortableness of passengers in trains. However, the higher reliability, which will guarantee a long life, is also required for the railcar power converters. For example, the reliability, which will guarantee the long life of more than about thirty years, is scheduled to be required for the next generation railcar power converters
FIG. 1 is a broken sectional view showing an example of an inner structure of a conventional plastic IGBT module used in such a power converter. A plastic side wall 2 is bonded to an edge of a metallic cooling plate 1. A plastic terminal cap 3 covers a top surface of this plastic side wall 2. A copper plate 5, which is directly bonded or silver-brazed to a bottom surface of a ceramic substrate 4, is soldered onto the metallic cooling plate 1 through a solder 6. A copper plate constituting an emitter wiring pattern 71, a collector wiring pattern 72 and a gate wiring pattern 73 is bonded on a top surface of the ceramic substrate 4. A semiconductor chip 8, such as IGBT chip and the like, is soldered to this copper plate 72 through a solder 13.
An emitter electrode pad on the surface of the semiconductor chip 8 is electrically connected to the emitter wiring pattern 71 by aluminum bonding wires 91, and a gate electrode pad is electrically connected to the gate wiring pattern 73 by an aluminum bonding wire 92. In addition, an emitter terminal 10, a collector terminal 11 and a gate terminal 12 which are made of copper are respectively soldered through solders 13 to the emitter wiring pattern 71, the collector wiring pattern 72 and the gate wiring pattern 73 and are erected upwards. Heads of the emitter terminal 10, the collector terminal 11 and the gate terminal 12 are protruding from the outer surface of the terminal cap 3, which supports and fixes the emitter terminal 10, the collector terminal 11 and the gate terminal 12. Moreover, in order to shield the semiconductor chip 8 from outside air, it is filled with a silicon resin 14, and an epoxy resin 15 is filled onto this silicon resin 14.
The semiconductor power modules used to control the system of driving the electric locomotive are required for the high reliability under the severe conditions of higher temperatures and higher humidities. The above mentioned conventional plastic IGBT module has the structure sealed with the silicon resin 14 or the epoxy resin 15. However, since this resin seal is of semi-seal structure, the conventional plastic IGBT module is of an incomplete sealed structure. Thus, the conventional plastic IGBT module is weak in moisture resistance. So, under the environment of the high temperature and the high humidity, water permeates into the module to thereby cause the performance deterioration of the semiconductor chip 8. This is undesirable in view of the reliability of a long time as the semiconductor power module for the electric locomotive. In addition, there may be a possibility that impurities (sodium (Na), chromium (Cr) and the like) will gradually be doped in the silicon resin used for the resin sealing. This impurities will invade the semiconductor chip 8 during the long operation. This results in a problem that the reliability may drop.
Moreover, outer members 2, 3 constituting the semiconductor power module are made of plastic. Thus, they are also weak in mechanical strength. This results in a problem that the explosion-proof durability is substantially null when the semiconductor chip 8 is exploded by a short circuit accident and the like.
Especially, some kinds of the electric locomotives, such as a suburban train, a subway transit car, a streetcar or the like, frequently repeat starts and stops. Thus, the semiconductor power module used therein are expected to have a very high power cycle durability. For example, a semiconductor power module for an railcar in a next generation is planned to have a high power cycle durability of about ten million times, in a junction temperature variation xcex94Tj=40xc2x0 C. and at a case temperature Tc=50xc2x0 C. However, in the above mentioned conventional plastic IGBT module, the semiconductor chip 8 and the wiring patterns 71, 72 and 73 made of the copper plates are connected to each other through the aluminum bonding wires 91, 92. Hence, the power cycle durability of the conventional semiconductor power module is, for example, only about one hundred thousand times at present. Therefore, it is difficult to satisfy the required power cycle durability for the next generation railcar.
Moreover, the difference of the thermal expansion coefficients between the metallic cooling plate 1 and the ceramic 4 is large, and the railcar frequently repeats the starts and the stops so as to cause severe temperature fluctuations. Then, the crack caused by the thermal stress due to the severe temperature fluctuations is induced in the solder 6. This results in a problem that a Thermal Fatigue Test (TFT) reliability and a Thermal Cycling Test (TCT) reliability of the conventional semiconductor power module are both low and insufficient. On the contrary, the semiconductor power module for the electric railcar in the next generation is planed to have a TFT reliability of about 50 thousands cycles at xcex94Tc=70xc2x0 C. (Tc=25xc2x0 C. to 95xc2x0 C.) and a TCT reliability of about 1000 cycles at xcex94Tc=165xc2x0 C. (Tc=xe2x88x9240xc2x0 C. to 125xc2x0 C.). However, a present semiconductor power module attains a low TFT reliability of about 5 thousands cycles and a low TCT reliability of about 100 to 300 cycles at the most, under the above mentioned conditions. This causes a problem that the planned specification will not be attained in the next generation.
On the other hand, various power converters, such as a DCxe2x80x94DC converter, a self-excitation inverter, a separate-excitation inverter and the like, are used in the respective control systems for driving the miscellaneous railcars. A rated specifications are variously changed depending on the type of railcar systems. For example, the suburban train requires a large scale module, which comprises a plurality of the semiconductor power modules, having a rated specification of a 800A, 3300 V class or a 1200A, 3300V class. The Japanese high speed train (referred as xe2x80x9cthe Shinkansenxe2x80x9d train in Japanese) requires the large scale modules having a rated specification of a 1200A, 4500V class. xe2x80x9cThe InterCityExpress (ICE)xe2x80x9d, the high speed train in Germany and Switzerland or xe2x80x9cthe Train a Grande Vitesse (TGV)xe2x80x9d, the high speed train in France requires the large scale modules having rated specification of 1200A, 6500V classes. Then, various large scale modules having diversified rated specifications are required. Also, changes of the specification of the large scale modules often occur depending upon the changes of system designs. However, in the conventional large scale modules assembling plurality of semiconductor power modules, it is not easy to change the maximum current handling capability or the maximum operating voltage. This results in a problem that the rated specifications of the large scale modules can not be changed rapidly and simply.
Thus, a large scale module was desired and required which could easily change the rated specifications, having structure that could be changed rapidly corresponding to the various specifications requested by users with low cost.
The present invention is proposed to solve the above mentioned problems.
That is, it is therefore an object of the present invention to provide a semiconductor power module strong in moisture resistance and to provide a large scale module, or an assembly of the semiconductor power modules.
Another object of the present invention is to provide a semiconductor power module having a large explosion-proof durability and to provide a large scale module comprising the semiconductor power modules.
Still another object of the present invention is to provide a semiconductor power module having a high TFT reliability and a high TCT reliability, and to provide a large scale module comprising the semiconductor power modules.
Still another object of the present invention is to provide a semiconductor power module having an excellent power cycle durability and to provide a large scale module comprising the semiconductor power modules.
Still another object of the present invention is to provide a large scale module which can adjust the number of semiconductor chips mounted in a package, or in the large scale module so as to change the power handling capability and the maximum operating voltage of the large scale module rapidly and easily, and further to provide the large scale module having a flexibility of rated performances so that it can correspond quickly to various specifications requested by users without wasting cost.
In order to attain the above mentioned objects, the first feature of the present invention lies in a semiconductor power module provided with: a ceramic substrate; a metallic plate bonded to a surface of this substrate; a cylindrical metallic flange which is hermetically bonded to a surface of the substrate at an outer circumference of the substrate, separated from the metallic plate; a disk-shaped ceramic housing for hermetically sealing an opening of this metallic flange; and at least one or more semiconductor chips mounted on and soldered to the metallic plate on the surface of the ceramic substrate. Here, as the semiconductor chip, it is possible to use a semiconductor chip merging various semiconductor switching devices, such as an IGBT, a power MOSFET, a power bipolar transistor (power BJT), a GTO thyristor, a power static induction transistor (power SIT), a static induction thyristor (SI thyristor) and the like. In addition, as the ceramic substrate, it is possible to use various substrates, such as alumina (Al2O3), aluminum nitride (AlN), silicon nitride film (Si3N4), beryllia (BeO) and the like.
According to the first feature of the present invention, a hermetically sealed container is created by the ceramic substrate, the cylindrical metallic flange and the disk-shaped housing. For example, one or more semiconductor chips, such as IGBTs and the like, are accommodated in this hermetically sealed container. And this hermetically sealed container has a very high air-tightness. For example, the air leak rate of less than 10xe2x88x924 Paxc2x7m3/sec (10xe2x88x925 atmxc2x7cm3/sec) is easily obtained by this hermetically sealed container. Further, it is practically easy for this hermetically sealed container to have the higher air-tightness (the lower air leak rate) of, for example, about 10xe2x88x928 Paxc2x7m3/sec (10xe2x88x929 atmxc2x7cm3/sec) to 10xe2x88x9210 Paxc2x7m3/sec (10xe2x88x9211 atmxc2x7cm3/sec). In addition, the ceramic substrate, the metallic plate made of, for example, copper, bonded to the surface thereof and the metallic flange are brazed with silver brazing, aluminum brazing or the like. Moreover, the brazed portion can be sufficiently strong against the thermal stress resulting from the difference of the thermal expansion coefficient between the ceramic substrate and the metallic plate, or between the ceramic substrate and the metallic flange.
The second feature of the present invention lies in a semiconductor power module provided with: a ceramic substrate; a metallic plate bonded to a surface of the ceramic substrate; a metallic flange which is hermetically bonded to a surface of the metallic plate at boundary of, or at an outer circumference of the metallic plate; a disk-shaped ceramic housing for hermetically sealing an opening of this metallic flange; and at least one or more semiconductor chips soldered to the metallic plate on the surface of the ceramic substrate. Here, as the semiconductor chip, it is possible to use the semiconductor chip merging the various semiconductor switching devices, such the IGBT, the power MOSFET, the power BJT, the GTO thyristor, the power SIT, the SI thyristor and the like, similarly to the first feature. In addition, as the ceramic substrate, it is also possible to use the substrate, such as the Al2O3, the AlN, the Si3N4, the BeO or the like, similarly to the first feature.
According to the second feature of the present invention, the cylindrical metallic flange is bonded to the metallic plate made of, for example, copper or the like, where the metallic plate is bonded on a surface of the ceramic substrate. Thus, if the metallic flange is made of metal with a low thermal expansion coefficient, the three layered structure in which the high thermal expansion coefficient material (the metallic plate) is sandwiched by upper low thermal expansion coefficient material (the metallic flange) and lower low thermal expansion coefficient material (the ceramic substrate) is constructed. Then, in the three layered structure, the thermal stress ascribable to the difference of the thermal expansion coefficient between the ceramic substrate and the metallic plate is reduced by the compensating stress acting between the metallic plate and the cylindrical metallic flange. Hence, it is possible to further improve the TFT reliability and the TCT reliability.
The third feature of the present invention pertains to a large scale module comprising a plurality of semiconductor power modules. Here, as the semiconductor power module, it is possible to use the semiconductor power module described in the above mentioned first and second features of the present invention. Namely, the large scale module of the third feature comprises: a heat sink; a metallic frame having a plurality of openings disposed on the heat sink; a plurality of semiconductor power modules disposed on the heat sink so as to be mounted in the openings; a plurality of sealing members disposed between the respective semiconductor power modules and the metallic frame; a plastic cover for covering one surface of the metallic frame on which the semiconductor power module is mounted; and a resin filled into the cover.
According to the third feature of the present invention, if a desirable number of openings are formed on the metallic frame, it is possible to mount a desirable number of semiconductor power modules in the metallic frame. For example, the parallel connection of a desirable number of semiconductor power modules enables the formation of the large scale module having a desirable maximum current handling capability and power handling capability at the maximum operating voltage. In addition, if a desirable number of semiconductor power modules are mounted in the metallic frame and further they are connected in series, it is possible to assemble the large scale module having desirable operating voltages, desirable breakdown voltages, or desirable blocking voltages. Moreover, the selection and the combination of the parallel connection and the series connection enables the formation of a large scale module having any power handling capability and the maximum operating voltages. Therefore, by adjusting the number of semiconductor chips mounted in the metallic frame and selecting/combing the parallel/series connection, it is easy to response to the various specifications of users rapidly without costing extra money.
Further it is easy to construct various inverters or various converters having desirable maximum current handling capabilities, the desirable power handling capabilities, or the desirable maximum operating voltages, rapidly responding to various specifications imposed by miscellaneous users.
Other and further objects and features of the present invention will become obvious upon an understanding of the illustrative embodiments about to be described in connection with the accompanying drawings or will be indicated in the appended claims, and various advantages not referred to herein will occur to one skilled in the art upon employing of the invention in practice.